With few exceptions the basic operating principles for electric motors and generators have not changed much over the past 100 years. With the development of high energy or high coercive force permanent magnets the power density and efficiency of electric motors was increased over the then state of the art motor technologies by replacing the field coils in brush motors or armature coils in brush-less motors with permanent magnets. The permanent magnets require less space and typically weigh less than the copper windings they replaced and reduce the I2R losses of the motor's total electrical system.
Replacing coils with permanent magnets introduced a new motor design challenge. The field of the permanent magnets cannot be ‘turned off,’ which introduces high cogging torques at start-up. The constant magnetic flux also causes the motor's back electromotive force to become linear with speed, resulting in a linear speed to torque relationship, which reduces the efficiency of operation when the motor is producing peak power. Most of the approaches to control the efficiency at peak output power for permanent magnet motors have been directed toward electronically controlling the phase excitation angles and current. This electronic control approach works well for modifying the linear speed to torque relation to produce a more hyperbolic speed to torque relationship, but requires increasing the size and ultimately the weight of the controlled motor. This controller approach results in a counter productive exercise for the most part because while permanent magnets were used to reduce motor size and weight, in order to optimize efficiency at peak power, the motor size and weight is increased to that of motors using copper windings. By having to resize the motor, some of the benefits of using permanent magnets in the motor are negated.